The term stretch hood refers to one form of packaging system in which a film tube is used to form a hood to bundle and protect goods. The goods may be a single object such as white goods (a washing machine or refrigerator) or electronic goods. The goods may be a collection of objects such as bottles, bags of soil, cement, fertilizer, polymer pellets or concrete blocks, bricks, tiles, insulation materials. The object or objects are often supported on a pallet or other supporting platform, referred to herein as a palletized load, to permit handling by a forklift device. The film tube is conveniently produced by blown film extrusion, which may involve coextrusion, to produce a tube from an annular die. Using blown film extrusion, the extruded tube is flattened for later use without slitting it longitudinally. Stretch hood relies on elastic contraction of the film tube around the object or collection of objects to be packaged, after a stretching device has placed the tube in a stretched condition around the object or collection of objects to be packaged to form the hood. The hood protects, holds the palletized load together against spillage if necessary and shields the palletized load from damage and environmental factors (moisture) during transportation and storage. The hood may be sealed at one end to protect and cover the top of the palletized load. Alternatively, the hood may be open at the top where protection is a lower priority.
Stretch hood packaging systems may provide certain advantages over other forms of packaging of palletized loads such as shrink hood, where a film tube is shrunk by the application of heat, or stretch wrap where a flat film is wrapped around the object or collection of objects to be packaged. In shrink hood a film containing a highly oriented polymer (often LDPE with optionally some LLDPE) is melt oriented upon extrusion and shrinks and thickens when shrunk around and onto a palletized load. Heat has to be applied to shrink the film into the position where it provides the holding force that holds load together against spillage. The process consumes significant energy and the use of heat creates safety debits. The film is relatively thick and stiff and need not be designed to stretch easily prior to heat shrinking. In stretch wrap, a more linear polymer (often linear low-density polyethylene LLDPE with optionally some VLDPE (an ethylene copolymer of lower density than the LLDPE) is extruded to provide the required stretching characteristics. A tackifying polymer or other ingredients must be added to establish sufficient a cling force on the surface. The film can cling to itself strongly when rolled up (referred to as blocking) and excessive noise may result when unrolling the film at high speed. The film is relatively thin and designed to stretch easily to considerable levels of over 200% for its application around a load. The holding force is provided by the contracting force exerted by the stretched film. Stretch wrap is applied as a flat, non-tubular film and may give incomplete protection against environmental factors.
This invention concerns inter-polymers, and multi-layer films, which are described in terms of their utility for stretch hood applications. However these polymers and films may have performance features that provide utility elsewhere. Appropriate uses might be stretch sleeve to apply a label to a bottle or agricultural films for silage or greenhouse film or for collation shrink applications.
The application of a film tube to package the object or collection of objects on a stretch hood packaging machine involves the steps described by way of example in EP0461667 for the case where the top of the stretch hood is sealed.
In a first step (see FIGS. 1 and 2 of EP0461667) a flattened film tube is unrolled and opened up to fit around a stretcher, which may be in the form of a frame as shown or in the form of four corner devices as illustrated in FIG. 6. At this stage the top of the tube can be heat sealed before it is cut-off, creating an inverted bag. The stretcher device can enter the inverted bag from below. The film material is gathered around the stretcher by take down rollers at each corner (not shown in the Figures). The film is gripped in a nip between the stretcher and the rollers. The takedown rollers cause the film to be folded transversely and gathered on the stretcher. The film and rollers have to have enough friction for an efficient gathering and take down operation. The heat sealing at the top end of the tube requires high hot tack and seal strength to survive subsequent stretching. The gathered, transversely folded tube is then expanded by the stretcher in the transverse film direction beyond the external dimensions of the palletized load. This requires a pre-determined elasticity that permits stretching and a reversion of the stretch later upon relaxation (see FIG. 3 of EP0461667). The expanded stretcher with the transversely stretched film tube is then passed downwards over the palletized load unfolding and releasing the film (see FIG. 4 of EP0461667). This requires that the film tube, in its tensioned condition, has a moderate coefficient of friction with the stretcher to allow it to be released easily from the stretcher while at the same time submitting the film to a sufficient force in the machine direction to achieve a moderate degree of machine direction stretch. The stretcher remains in the expanded state after releasing the lower edge of the film hood and returns upwards to the starting positions past the hood, sealed at the top, now contracted around the palletized load.
Different film tube structures have been suggested for stretch hood. WO00/37543 discloses on a three-layer film using a blend of a metallocene produced plastomer (page 8, lines 10 to 16) and a predominant amount of ethylene (E) vinyl acetate (VA) with a high amount of VA in a core layer and surface layers of an EVA with low VA content containing SiO2 as anti-block friction modifier. The term plastomer is used to indicate a low density copolymer of ethylene and α-olefins. The EVA polymers used in WO00/37543 are produced in a high-pressure process, generally in an autoclave reactor as described by way of example in EP0099646. These polymers generally come in the two types referred to in WO00/37543: low VA and high VA.
The use of high EVA limits the contracting force of the film hood. The film tends to be soft, is easily stretched but only exerts a limited force to return to its pre-stretched state. The high EVA is vulnerable to ambient heat and the film holding force may be weakened. These drawbacks can only be partially compensated by the addition of the plastomer. The surface friction provided by the low EVA skin layers, generally having a broader molecular weight distribution, can only controlled imprecisely by high amounts of anti-block, which has a negative effect on the transparency and mechanical properties.
Overall the film of WO00/37543 provides a weak holding force upon contraction after it has been stretched to a considerable degree in stretch hood packaging. The film thickness may have to be increased to obtain a sufficient holding force. Once stretched and contracted around a load, the hood may have a low puncture resistance and, once punctured, a poor tear propagation resistance, which can easily provoke a spill of the palletized load. Full stretching can also lead to wash boarding where different parts of the film stretch differently, giving rise to thicker and thinner film portions. Furthermore, the holding force can weaken when the palletized load is exposed to more elevated temperatures. Adjustment of the coefficient of friction for a particular stretch hood packaging machine or packaging purpose may be unduly limited in range and/or difficult to achieve without detracting from other desired film properties. Many stretchable, elastomeric polymers are inherently sticky. If such a polymer is selected for the surface of stretch hood film, high levels of anti-block may be needed to allow the heat-sealed top of the hood to flatten and fit closely around the pelletized load. Such anti-block levels may make the film opaque and interfere with the mechanical and optical properties.
U.S. Pat. No. 6,291,038 described a heat shrinkable film with a medium VA EVA having from 9 to 20 wt % of VA, and a narrow molecular weight distribution and high degree of short chain branching.
As used herein, short chain branching refers to alkyl branches, detectable through 13C NMR techniques and excludes acetoxy branches from the incorporation of vinyl acetate comonomer. Short chain branching as so defined in polymers made in high-pressure free-radical polymerization, results mainly from the Roedel backbiting mechanisms that lead to ethyl and butyl type branches as well as some higher levels of branching. Other short chain branches result from the incorporation along the chain of α-olefins which have generally the effect or the intent of lowering the molecular weight and are referred to as telogens or transfer agents. The use of propylene transfer agent results in methyl-type short chain branches. The use of isobutylene would also result in methyl-type short chain branches. Using NMR techniques the number of carbon atoms in the short chain branch can be determined and also the number of the respective types of short chain branches. Where reference is made 1000 C atoms as the basis for expressing the amount of SCB or LCB, the reference to the 1000 C atoms refers to all carbon atoms, including carbon atoms in acetoxy groups, unless otherwise mentioned.
Atofina markets a grade of low VA content EVA polymer: EVATANE (Registered Trade Mark) 1005VN3, which has a VA content of 3.5 mol % (9.8 wt %) and a Melt Index of 0.33. The film has limited short chain branching and a low relaxation time.
Accordingly, known film structures for stretch hood packaging limit the performance and savings obtainable from stretch-hood packaging systems. The range of applications in which it would perform competitively against stretch film or shrink hood packaging maybe unduly restricted.
It is among the objects of the invention to improve the performance of stretch hood packaging films. In particular it is among the objects to achieve a better balance between stretchability and holding force. It is among the objects to permit a) a reduction in the weight of film needed to provide a given holding force in stretch hood packaging; b) better friction control and a more even stretching of the film for the stretch hood packaging operation and/or c) improved package integrity and transparency, resistance to puncture and tearing, and/or reduced stress relaxation at higher ambient temperatures after the stretch hood packaging operation has been completed.